The book explores object oriented programming using the Eiffel language and one of the
things that I always loved about Eiffel was the defensive
programming techniques that it teaches and the strong focus on
contracts and preconditions (a technique that is used
extensively in Gnome).

Until very recently I had not noticed it, but the Eiffel
compiler and the Eiffel suite of libraries and even the
development IDE (EiffelStudio) were open sourced (Emmanuel
mentions this to me during the Lang.Net conference and I do
not remember any big news about it).